The invention relates generally to providing, by a cellular communication system, communication channels to terrestrial-based communication devices. More particularly, the invention relates to acquiring, synchronizing, and tracking communication devices using digital beamforming apparatuses and techniques in a satellite cellular communication system.
Current satellite cellular communication systems use either satellite-fixed or earth-fixed beams, within which communication channels are provided to users of the system. In a system using satellite-fixed beams, the beams projected by the satellite are fixed with respect to the satellite and move across the surface of the earth in accordance with the velocity vector of the satellite. Thus, in these systems, frequent handoffs of communication channels are necessary between beams of a particular satellite and between satellites.
In a system using earth-fixed beams, a satellite steers each beam toward a particular geographical region during the satellite""s pass over that region. Thus, even though the satellite is moving with respect to the surface of the earth, the beam footprints projected by that satellite do not move substantially with respect to the earth""s surface. Although handoffs between beams are less frequent in these systems, handoffs between satellites are necessary as one satellite passes out of view of a geographical region.
Each of these types of systems typically employs phased array antennas and digital beamforming techniques to steer the beams and/or to provide communication channels within the beams. The signal strength within each beam is typically of uniform distribution. Thus, with respect to signal power allocation, these systems are naturally inefficient, because relatively strong beams are provided in areas where few or no users exist, and the systems have no ability to target stronger signals toward areas where the users are. Therefore, it is desirable to increase signal strength in the direction of users, while minimizing signal strength in areas where no users exist.